Ultrafast confocal microscope for time-resolved imaging of thin films

A peculiar feature of organic semiconductors is structural inhomogeneity: in the solid state they are generally amorphous or poly crystalline, with local order only achieved in mesoscopic domains with size typically ranging from 10 nm to 1 mum. The dimensions and types of these domains have a decisive influence on excited state dynamics, which in turn determines fluorescence quantum yield, charge carrier mobility and generation efficiency, crucial parameters for optimizing the efficiency of organic optoelectronic devices. Femtosecond pump-probe spectroscopy provides a wealth of information on the photophysics of organic semiconductors. This paper presents a novel instrument which combines the spatial resolution of confocal microscopy with the temporal resolution and spectral sensitivity of femtosecond pump-probe spectroscopy using broadband detection. Thin films of poly(9,9-dioctylfluorene) (PFO), blended with polymethylmethacrylate (PMMA). PFO is a blue-emitting polymer, with an absorption maximum at 385 nm, while PMMA is optically inert at the given pump wavelength, are studied. Films with few-mum thickness are drop cast on a reflective metal substrate, so as to turn the DeltaR/R into a double-pass differential transmission. The presence of the metal surface does not affect the optical properties of PFO. The film shows a phase separation between PFO and the host matrix, with a large distribution of domain sizes ranging from sub-micron to hundreds of microns, making it an ideal sample to assess the performances of our ultrafast confocal microscope.